1. Field of the Invention
The present invention relates to electrical discharge machining apparatuses. In particular, the present invention relates to an electrical discharge machining apparatus which is preferably used for machining which requires very low roughness, such as precision machining using an electrical discharge.
2. Description of the Related Art
As a method for increasing working speed in electrical discharge machining, a method in which a plurality of discharges are simultaneously generated (hereinafter referred to as xe2x80x9cparallel dischargexe2x80x9d) is known. The method for producing the parallel discharge was initially used for preventing the surface smoothness in a large finishing-working region from deteriorating.
A method is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 61-71920 for improving the surface smoothness in a large finishing-working region by using electrodes with electrically resistant surfaces (hereinafter referred to as a xe2x80x9cresistant electrode methodxe2x80x9d).
FIG. 11 shows an electrode to be used in the resistant electrode method. The electrode shown in FIG. 11 includes a resistant element 1 made of a thin silicon plate having a thickness of 1.5 mm and a copper feed element 2 bonded to the resistant element 1 with a conductive adhesive.
The principle of the resistant electrode method is that stray capacitance formed in a gap between the electrode and a work (hereinafter referred to as a xe2x80x9cworking gapxe2x80x9d) is divided into a distributed parameter state by the resistance of the resistant element 1 provided at the end of the electrode, thereby reducing the amount of energy applied by the stray capacitance to the discharged area to the same amount of the energy applied when machining a small area, and thereby preventing the surface smoothness in a large machining region from deteriorating. The above-described disclosure refers to the fact that a plurality of discharges (parallel discharge) are generated due to a slight potential gradient produced in the electrode when a resistant element is provided at the surface of the electrode. However, in the resistant electrode method, a problem occurs, which is described below.
Resistant electrode methods similar to the method described above are disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 58-186532 and 62-84920, in which an electrode is divided into a plurality of columnar electrodes, thereby improving the smoothness of the machining surface of a large finishing-working region (hereinafter referred to as a xe2x80x9cdivided electrode methodxe2x80x9d).
FIG. 12 shows an electrode to be used in the divided electrode method. FIG. 13 is a perspective view of the entire configuration of the electrode to be used in the divided electrode method. The same or similar components shown in FIG. 11 are referred to with the same reference numerals and a description of those components is omitted. In FIGS. 12 and 13, insulative elements 3 and columnar members 4 made of a low-resistance material, such as copper, are shown.
The principle of the divided electrode method is that a plurality of the columnar members 4 are isolated from each other by the insulative elements 3 and are connected to the feed element 2 via the resistant element 1, as shown in FIG. 12, forming an electrode having the divided columnar electrodes in a bunched fashion, as shown in FIG. 13, thereby reducing the stray capacitance formed at the working gap to the level of stray capacitance formed when machining a small working area, and thereby preventing the machining-surface smoothness when working a large area from deteriorating. In the above-described disclosure of the divided electrode method, a parallel discharge is not referred to.
The parallel discharge is briefly mentioned in the Journal of The Japan Society for Precision Engineering, Vol. 53, No. 1, PP 124-130 in a description for the resistant electrode method, and not at all in the description for the divided electrode method. The parallel discharge referred to has the problem described below.
As described above, the countermeasure to overcome the problem of the surface-smoothness being deteriorated in finishing machining of a large area, which is adopted in the resistant electrode method, is to divide the stray capacitance produced at the working gap into smaller capacitances, thereby obtaining the same level of finishing-machining surface smoothness as that when machining a small area. That is, stray capacitance, which contributes to machining during discharging, is only produced in the vicinity of the region in which the discharge is generated (i.e., in a circle having a radius of several hundred microns), thereby suppressing the effect of the stray capacitance formed in the working gap, which causes a problem in the finishing machining of a large area.
In this case, the portion machined by the stray capacitance formed at the working gap is negligibly small, so that all the energy for machining can be considered to be supplied by a working current source in a pulsed manner. Therefore, it is considered that the energy to be supplied for working is substantially constant whether the parallel discharge is generated or not. The parallel discharge is generated at a plurality of discharge spots which are paths of flowing current divided from the working current supplied from the working current source. Therefore, the problem is that the discharge-machining speed is not increased by the parallel discharge.
Another problem is that the parallel discharge cannot be generated practically in the divided electrode method. This is because, in discharge machining, once discharge starts, subsequent discharges are continuously generated in the vicinity of the previous discharge because the distance between poles in which the discharge can be generated increases as the concentration of machined particles increases. It is considered that in a divided electrode including bundled columnar members forming, for example, a 10-mm square, as described in the above document, the discharges are generated continuously at the particular columnar member at which the initial discharge was generated. It is assumed that the reason why the parallel discharge in the divided electrode method is not described in the above document is because of this problem.
Another problem in the divided electrode method is that the structure of the electrode is complex.
Accordingly, it is a first object of the present invention to provide an electrical discharge machining apparatus in which the discharge machining speed can be increased.
It is a second object of the present invention to provide an electrical discharge machining apparatus in which parallel discharge is always generated and the configuration of the electrodes is simplified.
To these ends, according to an aspect of the present invention, an electrical discharge machining apparatus includes a discharge-machining electrode opposing a work across a working gap. The discharge-machining electrode includes a layered anisotropically conductive element including conductive layers and low-conductive layers alternately laminated on each other, a resistant element connected to one of layer-perpendicular end surfaces of the layered anisotropically conductive element, and a feed element connected to the resistant element. With this arrangement, the area of each conductive layer as a capacitor opposing the work can be increased, although the conductive layer is thin, by extending along the work surface. The capacitance formed in a working gap between the conductive layers and the work equals the sum of the small capacitors connected in parallel to each other through resistors. Each capacitor having a capacitance sufficient for discharge machining is disposed at a distance sufficiently close to the others for generating a parallel discharge, whereby machining can be performed by simultaneous and parallel electrical discharges, thereby increasing the discharge machining speed by using an electrode having a simple structure.
According to another aspect of the present invention, an electrical discharge machining apparatus comprises a discharge-machining electrode opposing a work across a working gap. The discharge-machining electrode includes a layered anisotropically conductive element including conductive layers and low-conductive layers alternately laminated on each other, a resistant element connected to one of layer-perpendicular end surfaces of the layered anisotropically conductive element, a feed element connected to the resistant element, and a conductive grounding element provided via a dielectric element on any one of the layer-perpendicular end surfaces of the layered anisotropically conductive element. The conductive grounding element is connected to the work. With this arrangement, the capacitance formed in a working gap between the discharge-machining electrode and the work can be further increased. Therefore, a simultaneous and parallel discharge can be generated more positively, and the discharge-machining speed can be further increased by using the discharge-machining electrode having a simple structure.
According to still another aspect of the present invention, an electrical discharge machining apparatus comprises a discharge-machining electrode opposing a work across a working gap. The discharge-machining electrode includes a layered anisotropically conductive element including conductive layers and low-conductive layers alternately laminated on each other, a resistant element connected to one of layer-perpendicular end surfaces of the layered anisotropically conductive element, and a feed element connected to the resistant element. The feed element includes at least two feed devices connected to the resistant element at positions thereof separated from each other in a direction parallel to the layer faces of the layered anisotropically conductive element, in a manner such that the difference in or the ratio of the opposing areas of the conductive layers of the layered anisotropically conductive element and the feed element between the feed devices differs depending on the individual conductive layers. The feed element includes current determining units for measuring electric current fed to the individual feed devices. With this arrangement, determination can be performed whether or not the charging current is concentrated to a specific capacitor, thereby preventing the machining surface of the work from being damaged.